A frequency converter is a device typically used to control a motor. FIG. 1 shows an example of a frequency converter connection. The frequency converter 20 is typically composed of two converters, a rectifier 21 and an inverter 22, between which is provided a direct voltage or a direct current intermediate circuit 23. The rectifier 21 and the inverter 22 may also be located physically separately, and one rectifier may supply a plurality of inverters via a common intermediate circuit 23. An example of a rectifier 21 is a diode bridge, which obtains its supply 40 from an alternating current source 10, which is for instance a 50 or 60-Hz alternating current network, and an example of an inverter 22 is an inverter bridge implemented by means of transistors (e.g. IGBT, Insulated-gate Bipolar Transistor) or other semiconductors. An inverter 22 is typically used to adjust the power transferred from the intermediate circuit 23 of the frequency converter to a motor 30. Accordingly, in the figure, the supply connection 50 between the inverter 22 and the motor 30 is for instance a three-phase alternating current connection, although the figures show the connection with one line for the sake of clarity. The control of the motor 30 with the inverter can be implemented reliably in such a manner that the motor implements accurately the desired speed or torque instruction, for example.
The frequency converter 20 typically comprises a protective diagnostics in case of internal malfunction of the frequency converter, which operate in such a manner that for instance when one branch of the semiconductor of one phase of the inverter 22 cannot be turned off (the branch shorts or gets erroneously continuous ignition regardless of the control), the diagnostics of the frequency converter detect the fault and stop the frequency converter.
The problem in the above-described arrangement is that the motor 30 is able to supply magnetization energy, i.e. what is known as reverse power via the branch of the faulty phase and the zero diodes of the other phases to the fault point, i.e. the short-circuit in the inverter 22, whereby, at worst, the semiconductor module may explode and cause great damage in the frequency converter. For example, the IGBT semiconductor module may be destroyed in said manner even during a time of less than 10 ms depending on the properties of the motor, the type of the semiconductors in the frequency converter, the voltage of the intermediate circuit 23, the output frequency at the fault moment and the position of the rotor of the motor 30 at the fault moment, for example.